FIG. 15 shows a conventional capacitance sensing system 1500 that includes sense electrodes (one shown as 1502), grounded electrodes (two shown as 1504-0/1), and a capacitance sensing circuit 1506. In the absence of a sense object 1508 (e.g., part of a body such as a finger, a stylus, or other conductive object) a capacitance Cp exists between the sense electrode 1502 and ground. The presence of a sense object 1508 introduces a capacitance Cf.
Schematic 1510 shows a capacitance Cx sensed by capacitance sense circuit 1506. Cf varies according to the proximity of a sense object 1508. In particular, Cx will grow bigger in the presence of a sense object 1508.
Conventional system 1500 includes a nonconductive touch surface 1512 serving as a touch surface. Non-conductive touch surface 1512 prevents sense objects (e.g., 1508) from touching a sense electrode (e.g., 1502). Absent such a nonconductive touch surface 1512, when a sense object 1508 makes direct contact with sense electrode 1508, because it is a conductor to ground, it can increase a capacitance between all other sense electrodes and ground, erroneously triggering touch indications for all other sense electrodes.
The above limitation has prevented capacitance sensing on a contiguous conductive surface.
Other conventional sensing systems have utilized sense methods other than capacitance sensing in combination with a conductive sense surface. As a first example, conventional systems have utilized piezoelectric sensors in contact with a conductive surface. In response to strain induced by touch events, piezoelectric sensors can generate an electric field. Drawbacks to piezoelectric sensors can include difficulty in tuning responses to customer's liking, susceptibility to radio frequency (RF) noise/interference (e.g., 800 MHz, 1.9 GHz signals can interfere with sense results), and cost of components, as piezoelectric systems can require higher precision analog-to-digital converters.
As a second example, mechanical buttons can include conductive surfaces. Drawbacks to mechanical buttons can be susceptibility to wear and tear from moving/contacting parts and dust/debris. Other drawbacks include the expense in making mechanical buttons waterproof or resistant. Further, for many applications, mechanical buttons can lack the aesthetics for a given design.